[belval]

As a general rule, most machines break during state changes like start/stop. Keeping an engine running means no piece is subjected to the extra acceleration of restarting and your starter isn't used.

[drcoopster]

You mean besides the constant vibration and acceleration of pistons changing direction multiple times each second?

[emmelaich]

Yep, which is why diesel cars auto-off/on when stopped is a mixed blessing.

Saves fuel but wears out the battery and starter motor much faster.